The invention relates generally to a foodware article having stick resistant properties, and more particularly to a foodware article having a multilayer, durable, stick resistant, ceramic coating, and to a method of making such foodware articles.
Cookware can be made a variety of materials, including cast iron, copper, aluminum, and steel. Each type of cookware has advantages and disadvantages.
Seasoned cast iron cookware has a tough, abrasion resistant surface. However, cast iron is subject to rusting, and it must be cleaned carefully to avoid damage to the cookware surface. In addition, acid in foods can cause leaching of the iron from the surface, which can lead to health problems in some cases.
Copper cookware has excellent heat transfer properties. However, it is easily scratched because it is much softer than other cookware materials, such as cast iron or stainless steel. Copper also oxidizes readily, which leads to tarnishing. Copper can be polished to remove the tarnish, but it requires substantial effort to maintain the surface finish. Copper ions can also leach into foods.
Stainless steel cookware is widely used. It is known for its strength and durability. Stainless steel is relatively easy to clean, and it holds its shine better than copper. However, food is more likely stick to stainless steel than to seasoned cast iron. Overheating, cooking with salt water, or letting the pan “cook dry” causes discoloration of the surface. In addition, although it is typically quite low, leaching of ions, such as iron, chromium, manganese, and nickel, can also be a concern with stainless steel.
Aluminum cookware has excellent heat transfer properties. However, aluminum is also subject to leaching of ions. One way to reduce this problem is to coat the surface of the aluminum. Anodized aluminum is coated with aluminum oxide. The oxide layer makes it much harder than untreated aluminum. (Untreated aluminum has a thin layer of aluminum oxide from reaction with oxygen in the air.) However, food will generally stick to anodized aluminum cookware unless oil is used in cooking. In addition, anodized aluminum cookware is not dishwasher-safe, as it can be discolored or corroded by typical automatic dishwashing products.
Aluminum can also be treated by thermal spraying to prevent leaching. However, this process produces a rough surface, and food will generally stick to the surface unless it is treated.
One well-known surface treatment for cookware involves the use of perfluorocarbon polymers. Perfluorocarbon coatings provide a non-stick surface, but they are easily scratched. Even though current perfluorocarbon coatings are tougher than their predecessors, they are still fairly easy to scratch. When the surface is scratched or nicked, flakes of the perfluorocarbon coating can get into the food being cooked. This flaking is objectionable to many people, despite the fact that perfluorocarbon flakes are not known to pose a health risk. In addition, although the perfluorocarbon polymers are safe at normal cooking temperatures, they can be damaged at high temperatures and may give off toxic fumes.
Another surface treatment involves the use of ceramic coatings. U.S. Pat. No. 5,447,803 describes the deposition of a layer of titanium and a layer of titanium nitride. The titanium nitride coating has high hardness, and a gold color. The titanium nitride coating can be oxidized or nitrided to stabilize the color, but these oxide or nitride coatings are thin and can still be scratched, resulting in possible discoloration of the pan.
U.S. Pat. No. 6,197,438 describes the use of a thick layer (about 2 to 50 microns) of chromium nitride or aluminum nitride as a primer or topcoat layer to achieve scratch resistance and non-stick properties. A decorative or functional top coat layer such as silicon nitride, alumina, or diamond-like carbon can be added. Ceramic coated foodware based on a plasma-sprayed aluminum alloy substrate is also disclosed.
U.S. Pat. No. 6,360,423 describes the deposition of a zirconium nitride coating on cookware. The surface must be polished to a high surface smoothness before the zirconium nitride layer is deposited in order to obtain a stick resistant coating. Although the zirconium nitride coating does not need to be oxidized or nitrided to stabilize the color, zirconium nitride can be discolored in varying degrees by overheating or by salty-based foods.
Therefore, there remains a need for a scratch resistant, color-stable, stick resistant ceramic coating having the appearance of metal, and suitable for use with both acidic and salty-based foods.